DNA replication in eukaryotic cells is tightly controlled and coordinated with other events during the cell cycle and cell proliferation. Studies carried out with the budding yeast, Saccharomyces cerevisiae, led to the identification of an origin recognition complex ("ORC") (Bell, S. P., et al., "ATP-dependent Recognition of Eukaryotic Origins of DNA Replication by a Multiprotein Complex,"Nature, 357:128-34 (1992)). ORC consists of six polypeptides (i.e. ORC1 to ORC6) and plays an essential role in the initiation of chromosome DNA replication and transcriptional silencing in the budding yeast (Bell, S. P., et al., "Yeast Origin Recognition Complex Functions in Transcription Silencing and DNA Replication," Science, 262:1844-49 (1993); Bell, S. P., et al., "ATP-dependent Recognition of Eukaryotic Origins of DNA Replication by a Multiprotein Complex," Nature, 357:128-34 (1992); Micklem, G., et al., "Yeast Origin Recognition Complex is Involved in DNA Replication and Transcriptional Silencing," Nature, 366:87-89 (1993)). This six-protein complex binds to several well defined autonomously replicating sequences that serve as the chromosomal replication origins in an ATP-dependent manner (Bell, S. P., et al., "ATP-dependent Recognition of Eukaryotic Origins of DNA Replication by a Multiprotein Complex," Nature, 357:128-34 (1992)). The genes for all six ORC subunits have been isolated and deletion of any one of these genes results in lethality (Bell, S. P., et al., "Yeast Origin Recognition Complex Functions in Transcription Silencing and DNA Replication," Science, 262:1844-49 (1993); Bell, S. P., et al., "The Multidomain Structure of Orc 1 p Reveals Similarity to Regulators of DNA Replication and Transcriptional Silencing," Cell, 83:563-68 (1995); Foss, et al., "Origin Recognition Complex (ORC) in Transcriptional Silencing and DNA Replication in S. cerevisiae," Science, 262:1838-44 (1993); Li, J. J., et al., "Isolation of ORC6, a Component of the Yeast Origin Recognition Complex by a One-Hybrid System," Science, 262:1870-74 (1993); Loo, S., et al., "The Origin Recognition Complex in Silencing, Cell Cycle Progression, and DNA Replication," Mol. Biol. Cell, 6:741-56 (1995); Micklem, G., et al., "Yeast Origin Recognition Complex is Involved in DNA Replication and Transcriptional Silencing," Nature, 366:87-89 (1993)). The regulation of initiation of metazoan chromosomal DNA replication is poorly understood, because chromosomal origins have not been localized to specific DNA sequences. Recently, cDNA clones encoding proteins homologous to the S. cerevisiae ORC have been isolated from Schizosaccharomyces pombe, Xenopus laevis, and human for both ORC1 and ORC2, from kluyveromyces lactis for ORC1, and from Caenorhabditis elegans, Arabidopsis thaliana and mouse for ORC2 (Carpenter, P. B., et al., "Role for a Xenopus Orc2-related Protein in Controlling DNA Replication," Nature, 379:357-60 (1996); Gavin, K. A., et al., "Conserved Initiator Proteins in Eukaryotes," Science, 270:1667-71 (1995); Leatherwood, J., et al., "Interaction of Cdc2 and Cdc18 with a Fission Yeast ORC2-like Protein," Nature, 379:360-63 (1996); Muzi-Falconi, M., et al., "Orp1, a Member of the Cdc18/Cdc6 Family of S-phase Regulators, is Homologous to a Component of the Origin Recognition Complex," Proc. Natl. Acad. Sci. USA, 92:12475-79 (1995); Rowles, A., et al., "Interaction Between the Origin Recognition Complex and the Replication Licensing System in Xenopus," Cell, 87:287-96 (1996); Takahara, K., et al., "Mouse and Human Homologues of the Yeast Origin of Replication Recognition Complex Subunit ORC2 and Chromosomal Localization of the Cognate Human Gene ORC2L," Genomics, 31:119-22 (1996)). The Orc2 and Orc5 homologues have also been cloned from Drosophila melanogaster (Gossen, M., et al., "A Drosophila Homolog of the Yeast Origin Recognition Complex," Science, 270:1674-77 (1995)). A multisubunit protein complex made up of subunits homologous to the S. cerevisiae ORC genes has been purified from Drosophila and Xenopus (Gossen, M., et al., "A Drosophila Homolog of the Yeast Origin Recognition Complex," Science, 270:1674-77 (1995); Rowles, A., et al., "Interaction Between the Origin Recognition Complex and the Replication Licensing System in Xenopus," Cell, 87:287-96 (1996)).
Although origin of recognition complex subunits have been isolated and sequenced in some species, including human (i.e. ORC1 and ORC2), the need remains for such work to be carried out for other subunits. The present invention is directed to fulfilling this objective.